The present invention relates to decorative titanium material that is hardened on its surface and therewithin, and to a method of hardening such a titanium material.
In recent years, titanium and titanium alloys have come to be used in variety of fields, making use of the light weight and rustless of these materials, and the fact that they do not produce allergic reactions.
These features are particularly effective when these materials are used as materials for wrist watches, and been the subject of applications in this field in the past.
Titanium and titanium alloys, however, do have the drawback of being intrinsically susceptible to surface damage. Because such applications as mirror-surface finishing to achieve an attractive appearance would mean that damage to the surface would be visually apparent, in the past these materials have been subjected to sandblasting or the like so that damage is not readily apparent.
For this reason, the general public has developed an image of titanium and titanium alloys as having a dull surface when used as a decorative material.
The phenomenon of being easily damaged is attributed to a low surface hardness, and a variety of types of hardening have been performed with respect to titanium.
Methods of surface-hardening titanium can be divided into two main types: those which coat the titanium material surface with a hard film, and those which harden the titanium material itself.
Known methods of coating the titanium surface with a hard film include such wet processes as electroplating, and such dry processes as vacuum deposition, ion plating, sputtering, and plasma CVD. All of these methods, however, have problems with regard to achieving an intimate attachment to the material, and have not been developed to the point of solving the problem of film peeling.
Known methods of hardening the titanium material itself include ion implantation, ion nitriding, gas nitriding, gas carburizing, and gas soft nitriding. Because these methods, however, require a long processing time they present a problem with regard to productivity, and because of the high processing temperature used with these methods, the crystal grains become coarsen, causing surface roughness, this presenting problems with regard to a deterioration in a quality of outer appearance, and limiting the scope of usefulness.
As a result, for surfaces of wrist watches, eyeglasses, and accessories, in which an attractive appearance is required, it was not possible in the past to maintain the surface roughness that was achieved before hardening after hardening is performed.
Of the above-noted methods, because the method of hardening the titanium material itself results in a gradient of concentration of a diffused element from the surface within the metal, there is no problem with film peeling, and this method is thought to be effective as a method of surface-hardening titanium material.
However, there is still the problem of a deterioration in the quality of appearance caused by a surface roughness.
In ion nitriding technology, to reduce the degree of surface roughness, a method that has been used is that of reducing the sputtering effect. However, there has not been a basic reduction in the surface roughness caused by diffusion of nitrogen, carbon, or oxygen into the material itself.
Thus, in methods such as gas nitriding, carburizing, and oxidation for hardening the titanium material itself, the prior art did not include, as a method of reducing the surface roughness, such approaches as performing preprocessing to change the surface roughness of the material itself before processing, and did not envision attention to be paid to the size of crystal grains of the metal material itself, or the size of the crystal grains that grow in a planar direction on the hardened surface.
The problem of deterioration in quality of appearance is thought to be particularly attributable to a surface roughness caused by protrusions at the crystal grain boundary occurring at the initial phase.
Protrusions at the crystal grain boundary which occur in gas nitriding and in oxidization and nitriding are thought to be caused by stress concentrations at the crystal grain boundary that are caused by the formation of compounds at the crystal grain boundary or by lattice distortion caused by solid solution of nitrogen and oxygen.
If the protrusions at the crystal grain boundary are observed on a visual observation, a roughening of the surface can be perceived, this in particular making application impossible for use of the titanium material as a decorative material with a mirror polished.
As the height of these protrusions increases, the maximum height Rmax and mean surface roughness Ra increase, and the quality of the appearance deteriorates.
It has been discovered that the height of the protrusions at the crystal grain boundary is attributed to the size of the crystal grains in the titanium material before processing, and that the height of the protrusions becomes larger, the larger are the crystal grains that grow in the planar direction after hardening of the titanium material or the larger are the crystal grains before hardening.
In gas nitriding as done in the past, because heating is done to a temperature that is close to the transformation point (850xc2x0 C. to 870xc2x0 C.), a phenomenon of the crystal grains become coarse occurs and, from the above considerations, there is a further enlarging of the protrusions at the crystal grain boundary.
In particular in the case of a decorative metal material using either titanium or a titanium alloy, with gas nitriding as done in the past, because heating is done to a temperature that is close to the transformation point (800xc2x0 C. to 870xc2x0 C.), the crystal grains become coarse, and a stress concentration occurs at the crystal grain boundary, caused by the formation of compounds at the crystal grain boundary or by lattice distortion caused by solid solution of nitrogen, oxygen or carbon, this causing protrusions at the crystal grain boundary.
The height of these protrusions is larger, the larger is the higher of the size of the crystal grains of titanium or titanium alloy itself before processing. When viewed on a visual observation, there is a perception of a surface roughness, this leading to the problem of not being able to use this material, in particular, as a decorative material having a mirror polished.
That is, in a method such as gas nitriding, carburizing, oxidation, or nitriding, in which the titanium material itself is hardened as was done in the past, it was not possible to solve the problem of deterioration of the appearance, that is, to solve the problem of surface roughness of the material after the hardening process.
Accordingly, it is an object of the present invention to solve the problems accompanying the above-noted prior art, by providing a hardened titanium material that does not exhibit a deterioration in appearance even after hardening, and exhibits little surface roughness.
In order to achieve the above-noted object, a hardened titanium material and method of hardening a titanium material according to the present invention has the following technical constitution.
Specifically, the present invention is a decorative titanium material 2 which has a hardened layer 20 over a titanium material 21, the hardened layer 20 on the surface includes nitrogen and oxygen, and also the size of the crystal grains 24 at the surface of this decorative titanium material 2 (the diameter indicated as 26 in FIG. 1) is in the range from 0.1 to 60 xcexcm, and the maximum height of the surface roughness Rmax of the decorative titanium material 2 is no more than 1000 nm.
A method of hardening a decorative titanium material according to the present invention has a step of heating so as to raise the temperature of the titanium material in an inert gas atmosphere, a first hardening step of heating the titanium material in a first atmosphere, which is an atmosphere that includes nitrogen and oxygen, to a processing temperature of at least 700xc2x0 C. , a second atmosphere adjustment step of heating the titanium material in an inert gas atmosphere of argon or helium or the like to a processing temperature of at least 700xc2x0 C. , and a step of cooling the titanium material in an inert gas atmosphere.
Another aspect of a method of hardening a titanium material according to the present invention has a step of forming a protective film 10 that has a fine crystal grain size 24 in the range from 0.1 to 60 xcexcm onto the surface of a decorative titanium material 2, a step of heating the titanium material with a raising temperature in an inert gas atmosphere, a first hardening step of heating the material to a temperature of at least 700xc2x0 C. in an atmosphere that includes oxygen and nitrogen, as the first atmosphere, a second atmosphere adjustment step of heating the titanium material in an inert gas atmosphere of argon or helium or the like to a processing temperature of at least 700xc2x0 C. , and a step of cooling the titanium material in an inert gas atmosphere.
Yet another aspect of a method of hardening a decorative titanium material having a hardened surface layer according to the present invention has a step of forming a protective film having a crystal grain size in the range from 0.1 to 60 xcexcm onto the surface of the decorative titanium material, a step of heating the titanium material with a rising temperature in an inert gas atmosphere, a first hardening step of heating the material to a temperature of at least 700xc2x0 C. in an atmosphere that includes oxygen and nitrogen, as the first atmosphere, a second atmosphere adjustment step of heating the titanium material in an inert gas atmosphere of argon or helium or the like to a processing temperature of at least 700xc2x0 C., and a step of cooling the titanium material in an inert gas atmosphere.
In a hardened titanium material obtained by the decorative titanium material hardening method of the present invention, by making the crystal grain size after processing be in the range from 0.1 to 60 xcexcm, or by a step of forming a protective film thereonto which has microfine crystal grains, it is possible to eliminate the deterioration of the appearance after processing, that is, it is possible to obtain a surface with little roughness.
It is clear that the problem with deterioration of the appearance with regard to the present invention is attributed to a surface roughness caused by protrusions at the crystal grain boundary 22 in the initial phase.
The protrusions in the crystal grain boundary 22 that occur during processing by gas nitriding, oxidation and nitriding or the like are thought to be caused by stress concentrations at the crystal grain boundary that are caused by the formation of compounds at the crystal grain boundary or by lattice distortion caused by solid solution of nitrogen or oxygen.
If the protrusions at the crystal grain boundary 22 are observed visually, a surface roughness is perceived, this presenting a particular problem, in that use of the material is not possible as a decorative material with a mirror polished.
As the height of these protrusions increases, the maximum height of surface roughness Rmax and mean surface roughness Ra increase, and the quality of the outer appearance deteriorates. In the present invention, it was discovered that the height of the protrusions at the crystal grain boundary is attributed to the size of the crystal grains in the titanium material itself before processing, and that the height of the protrusions becomes larger, the larger are the crystal grains of the titanium material.
In the case of using titanium or a titanium alloy as a decorative metal material, protrusions occur at the crystal grain boundary, these occurring due to stress concentration at the crystal grain boundary because of the formation of compounds such as titanium nitride (TiN) and titanium oxide (TiO2) at the crystal grain boundary, or lattice distortion caused by solid solution of nitrogen and oxygen.
The larger is the crystal grain size of the titanium or titanium alloy before processing, the greater will be the height of the above-noted protrusions.
When this is viewed on a visual observation, it is permitted as a roughening of the surface, this leading to a deterioration of the appearance, making the material unusable in particular as a decorative material with a mirror polished.
Additionally, after processing, as the formation of compounds such as titanium nitride (TiN) proceeds at the crystal grain boundary and within the grains, this phenomenon can be observed as a surface roughness of the surface on a macro level, this also representing a deterioration of the outer appearance that makes the material unusual in particular as a decorative material with a mirror polished.
By using a titanium material having a surface with crystal grains having sizes in the range from 0.1 to 60 xcexcm, and performing heat treating under controlled temperature and time conditions in an atmosphere that includes nitrogen and oxygen, by virtue of the effect of a small crystal grain size before heat treating and the effect of nitrogen and oxygen that are solid solution into the crystal grain boundary inhibiting the coarsening of the crystal grains, it is possible to maintain crystal grains that grow in a planar direction with a size of 0.1 to 60 xcexcm while performing the processing.
When the above-noted processing is performed, the height of the protrusions at the crystal grain boundary reduced. That is, stress at the crystal grain boundary that occurs because of the lattice distortion caused by the solid solution and diffusion of nitrogen and oxygen is distributed by the effects such as an increase in the proportional of unit surface area occupied by the crystal boundaries.
As a result of this phenomenon, there is a reduction in the surface roughness which, when viewed on a visual observation, makes it possible to inhibit the deterioration in the appearance of the material.
In the present invention, by forming a protective film having a crystal grain size in the range 0.1 to 60 xcexcm onto the surface of a decorative titanium material and then performing heat treating thereof in an atmosphere of nitrogen and oxygen, the effects of a microfine crystal grain size before heat treating and the inhibition by nitrogen and oxygen of a roughening of the crystal grain size are achieved, making it possible to maintain surface crystal grain size that grow in a planar direction with a size of 0.1 to 60 xcexcm while performing the processing.
In this case, for the same reasons described above, the height of the protrusions at the crystal grain boundary is reduced.
That is, as shown in FIG. 5, using a titanium material that has a large crystal grain size of its surface, when hardening is performed the crystal grains become enlarged, resulting in protrusions at the crystal grain boundaries.
However, as shown in FIG. 4, if hardening is performed of a titanium material having a surface with small crystal grain size, the crystal grain size after processing are also small, and it can be seen that the protrusions at the crystal grain boundaries are reduced in size as well.